1. Field of the Invention
The present invention relates to a television tuner, and more particularly to a television tuner which can receive signals without high voltage applied to variable capacitors and uses an integrated circuit to simplify the structure.
2. Description of the Related Art
The standard structure of a conventional television tuner is as illustrated in FIG. 4: an input tuning circuit 31, a high frequency amplifier 32, an interstage tuning circuit 33, a mixer 34, an oscillator 35 for supplying a local oscillation signal to the mixer 34, a PLL circuit 36 for applying tuning voltage to the input tuning circuit 31, interstage tuning circuit 33 and oscillator 35, and a DC-DC converter 37 for supplying high voltage (30 V or so) to the PLL circuit 36 are connected in cascade.
The input tuning circuit 31 is made up of a single tuning circuit which incorporates an inductor 31a and a varactor diode 31b. According to tuning voltage TU (from approx. 2 V to 27 V) applied to the varactor diode 31b, tuning is done to a television signal from a channel to be received. As the tuning voltage varies in the above range, the capacitance value varies from approx. 2 pF (picofarads) to 20 pF. The high frequency amplifier 32 amplifies the tuned television signal. The interstage tuning circuit 33 is a double tuning circuit which is composed of a primary tuning circuit having an inductor 33a and a varactor diode 33b and a secondary tuning circuit having an inductor 33c and a varactor diode 33d. The same tuning voltage TU is applied to the varactor diodes 33b and 33d in the interstage tuning circuit 33 so that tuning is done to a television signal from a channel.
The television signal selected by the interstage tuning circuit 33 enters the mixer 34 where it is mixed with a local oscillation signal and converted into an intermediate frequency signal. Hence, the local oscillation frequency follows the tuning frequencies of the tuning circuits 31 and 33 in a way to stay higher than them by just as much as the frequency of the intermediate frequency signal. This is what is called “tracking.” The oscillator 35 which supplies a local oscillation signal has a resonant circuit which incorporates an inductor 35a and a varactor diode 35b. The characteristics of this varactor diode 35b are the same as those of the varactor diode 31b of the input tuning circuit 31 and the varactor diodes 33b and 33d of the interstage tuning circuit 33. The same tuning voltage TU is applied to the varactor diode 35b. 
The PLL circuit 36 which supplies the tuning voltage requires high voltage (30 V). This high voltage is supplied from the DC-DC converter 37. The PLL circuit 36 and the DC-DC converter 37 are included in an integrated circuit 38. The DC-DC converter 37 boosts the supply voltage B (5 V or so) applied to the integrated circuit 38 by double rectification and supplies it to the PLL circuit 36. The PLL circuit 36 generates a tuning voltage TU which ranges from approx. 2 V to 27 V, according to input channel selection data.
However, when a required high voltage as a tuning voltage is generated by the DC-DC converter, the withstand voltage for the integrated circuit as a restrictive factor must be taken into consideration. In addition, recently there has been a tendency to use low supply voltages for television tuners and their integrated circuits. Under these circumstances, it is becoming more and more difficult for a DC-DC converter to generate a high tuning voltage.
One possible solution to this problem is to use varactor diodes whose capacitance largely varies even at low voltage. However, this type of varactor diode has a problem: it is likely to cause distortion due to a strong electric field caused by a TV signal.